1. Field of the Invention
This invention relates to a process for regenerating solid disproportionation catalysts by contacting first with alkenes and then with an oxygen containing gas.
2. The Prior Art
The catalytic disproportionation of alkenes is a reaction described, for example, in an extensive review article by G. C. Bailey in "Catalysis Reviews"3 (1), 37-60 (1969). Disproportionation processes are also noted in U.S. Pat. No. 3,365,513 issued Jan. 23, 1968; U.S.Pat. No. 3,444,262 issued May 13, 1969; U.S. Pat. No. 3,579,602 issued May 18, 1971; U.S. Pat. No. 3,660,507 issued May 2, 1972; U.S. Pat. No. 3,725,496 issued Apr. 3, 1973 and U.S. Pat. No. 3,726,930 issued Apr. 10, 1973. A wide variety of solid materials are active disproportionation catalysts, for example supported compounds of titanium, vanadium, chromium, manganese, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, tin, hafnium, tantalum, tungsten, rhenium, osmium and iridium. Supported compounds of molybdenum, tungsten and rhenium are most suitable. The disproportionation catalysts may contain one or more alkali metal compounds for suppressing side reactions, such as isomerization and polymerization. Compounds of potassium, rubidium and/or cesium giving an alkaline reaction, in an amount between, for example, 0.5-5% by weight, calculated as alkali metal on the support are used for this purpose.
As a rule the activity of a solid disproportionation catalyst gradually declines during use, and the catalyst, after some time, must be regenerated or replaced by fresh catalyst. When deposition of organic material on the catalyst is the cause of this activity drop, the regeneration can be performed by treating the catalyst with an oxygen-containing gas to burn off the organic material (cf. U.S. Pat. No. 3,365,513, issued Jan. 23, 1968). A disadvantage of this regeneration method is that the catalyst may be damaged as a result of too rapid heating of the catalyst particles of exposing the catalyst to temperatures above about 650.degree. C. It is feasible to avoid the use of these high temperatures by treating the catalyst with a gas having a low oxygen content, for example between 0.5 and 4%v. An example of such a gas is air diluted with a gas inert to the catalyst at regeneration conditions, such as nitrogen, a noble gas or steam. However, this method is energy- and time-consuming. The applicant has found that those problems caused by these prior art methods of catalyst regeneration are minimized when a considerable part of the deposited organic materials are removed by contact with alkenes prior to the burn-off with an oxygen containing gas.